It is known that the illumination of a fiber end by a signal-modulated light source, such as a light-emitting diode, can be made more efficient if the rays from that source are concentrated with the aid of an interposed collective lens. The same effect can be had if the fiber end itself is given a spherical shape so as to act as a focusing means.
In an article entitled "Design Criteria For Optimum Coupling Between a High-Radiance LED and a Bulb-Ended Fiber", published by our assignee in CSELT RAPPORTI TECNICI Volume VI, No. 4 dated December 1978, we have described the results of a study carried out with bulb-ended (i.e. convex-faced) optical fibers having different radii of curvature in a range of 45 to 100.mu..
Such a curvature can be achieved by utilizing the surface tension of the vitreous material from which the fiber is made. Thus, the fiber end to be shaped is heated to a temperature near its melting point whereby its radius of curvature is caused to pass rather rapidly from infinity to a minimum value, followed by an increase. Since only a small deviation from the optimum radius can entail a considerable loss in transfer rate, careful control of the operation is required to insure a prompt termination of the heating process when the desired curvature has been reached.